From DE 10 2005 002 337 A1 a transmission device designed as an 8-gear multi-step transmission is known, which comprises frictional shifting elements such as disk clutches and disk brakes. When a shift demand is made for a gearshift in the transmission device, at least one frictional shifting element which is engaged in order to obtain the actual gear currently engaged in the transmission device has to be disengaged out of the force flow of the transmission device, whereas at least one other frictional shifting element, which is disengaged from the force flow of the transmission device while the actual gear is currently engaged in the transmission device, has to be engaged in the force flow of the transmission device in order to obtain the required gear.
During this, with increasing shifting time the torque transmitted by the frictional shifting element engaged in the force flow in order to obtain the current actual gear of the transmission device is transferred to a greater or lesser extent to the frictional shifting element that has to be engaged in the force flow of the transmission device in order to obtain the required target gear, while the torque transmitted by the shifting element to be disengaged decreases.
Disadvantageously, frictional shifting elements in the open operating condition give rise to drag torques which impair the overall efficiency of an automatic transmission to an undesired extent.
For that reason transmission devices such as that known from DE 10 2008 000 429 A1 are to an increasing extent designed, besides frictional shifting elements, also with interlocking shifting elements in the area of which no drag torques that impair the overall efficiency of a transmission device occur.
In that case, however, it must be borne in mind that interlocking shifting elements can only be shifted when close to their synchronous point from an open operating condition, in which no torque can be transmitted by the interlocking shifting element, to their closed operating condition, in which the full torque applied can be transmitted by the interlocking shifting element. In addition, interlocking shifting elements engaged in the force flow of a transmission device can only be disengaged from the force flow by means of low shifting forces when they are close to their load-free operating condition. In contrast to frictional shifting elements, both for the synchronization of interlocking shifting elements and also to change interlocking shifting elements to their load-free condition, additional constructional devices are needed in order to enable shifts in transmission devices that involve at least one interlocking shifting element to be carried out within desirable shifting times.
With the transmission devices described above, in order to be able in each case to carry out a required gearshift from an actual gear in the direction of a target gear, to implement which an interlocking shifting element has to be changed from its closed to its open operating condition while at the same time a frictional shifting element has to be changed from its open to its closed operating condition, and to do this to a desired extent with good shifting quality and a high level of shifting comfort, the interlocking shifting element to be disengaged must be opened at the correct time, i.e. within an operating condition window during which the torque applied to the interlocking shifting element is substantially at least approximately equal to zero.
For example, if the opening of the interlocking shifting element is mistimed due to too early or too late actuation of the interlocking shifting element, there is a possibility that at a predefined nominal opening time at least some of the torque transmitted by the transmission device will still be applied at the interlocking shifting element, and due to the opening of the interlocking shifting element this will result, in a vehicle drive-train made with the transmission device, in undesired drive-train reactions caused by the abrupt fall of the torque applied at the interlocking shifting element, which are perceived by a driver of a vehicle built with the vehicle drive-train as an unpleasant and appreciable jerk.
Moreover, if a disengagement or separation attempt is made too late when the transmission capacity of the frictional shifting element to be engaged is already too high, the possibility exists that the interlocking shifting element can only be changed to its open operating condition by applying undesirably large shifting forces, this being because of high friction forces between the mutually engaged shifting element halves of the interlocking shifting element to be opened, caused by stresses in the area of the transmission device.
If the interlocking shifting element to be disengaged is not changed to its open operating condition to the desired extent and at the same time the frictional shifting element to be engaged is actuated in the direction of its closed operating condition, undesired redundancies occur in the area of a transmission device which in some circumstances have the result that owing to a too rapid or defective build-up of the transmission capacity in the area of the frictional shifting element to be engaged, from a defined time point onward an interlocking shifting element to be disengaged can no longer be changed to its open operating condition within predefined operating times.
In transmission devices made exclusively with frictional shifting elements, by virtue of a software-technical evaluation of the actuating currents of electrically operated actors of an electro-hydraulic actuating device it is ensured that redundancy of a gearset is avoided. With an appropriate design of the frictional shifting elements, for example in which the frictional shifting elements are in each case acted upon in the opening direction by a spring mechanism that applies an opening force, it can be assumed that, provided the electrically operated actors or pressure regulators are correctly controlled, in the event of an opening demand the frictional shifting elements will in fact change to their open condition also because of the spring preloading.
In the case of interlocking shifting elements, for the reasons described earlier this cannot be ensured over the full operating range of an interlocking shifting element.